OFF NIGHT: With the Mets watching his everymove, ZackWheeler delivers on the way to giving up four runs on eight hits in 4-plus innings for Triple-A Las Vegas last night. Photo: John Patzer

TACOMA, Wash. — Zack Wheeler, cruising to an easy night here at Cheney Stadium, lost his rhythm after the Las Vegas 51s put up a long, five-run top of the fifth inning.

So on the bright side, that kind of run support won’t be a problem once he joins the Mets.

In what was probably his next-to-last start before making his long-awaited Mets debut, the prized right-hander failed to secure the win for himself in what wound up a 12-5 Las Vegas victory over Tacoma (the Mariners’ Triple-A affiliate.

Wheeler threw 38 pitches and allowed three runs in the fifth inning, getting two outs, (one out short of qualifying for a win) before manager Wally Backman lifted him. Wheeler’s final line produced 4 2/3 innings, eight hits, four runs (all earned), a walk and seven strikeouts. In all, he threw 105 pitches, 65 of them for strikes.

“I knew what I was doing wrong [in the fifth], but I couldn’t make the adjustment, which stinks as a pitcher,” Wheeler said. “You’re expected to make adjustments when you’re out there.”

The problem, Wheeler said, was that he was “falling down” a little bit with his front side. Despite a visit from Mets pitching coach Randy St. Claire and two from his catcher Francisco Pena, he couldn’t correct the mechanical flaw.

“You get hurt when you pitch up in the zone,” Backman said. “I thought he pitched a good ballgame until [the fifth], and then he pitched the ball up in the zone.”

Wheeler’s final-inning slip-up, which raised his 2013 ERA to 4.14, almost certainly will not impact the Mets’ plans for him. Though they had discussed calling him up for next weekend’s home series against the Cubs, the Mets’ two rainouts in two days appears to have altered the organization’s schedule for Wheeler. The Georgia native now is most likely to start one of the Mets’ two games in Atlanta on June 18 as his first big-league outing.

“He’s getting close,” Backman said before the game. “His timetable is definitely running short here at Triple-A.”

Six of Wheeler’s seven strikeouts came on his fastball, which reached as high as 99 miles per hour on the ballpark’s radar gun (which was fixed in anticipation of Wheeler’s arrival). The other came on a curveball. He recorded 1-2-3 innings in the first and fourth and escaped a second-and-third, two-out jam in the second when he retired Carlos Triunfel on a groundout to first base.

After the 51s blew open the game, however, sending nine batters to the plate, Wheeler couldn’t find his way. He walked leadoff batter Rich Poythress in a nine-pitch at-bat, featuring a close ball three, and Triunfel punched a single to right field. Abraham Almonte struck out, but Brad Miller ripped a single to right field, scoring Poythress.

Recently demoted Dustin Ackley doubled home Triunfel, Carlos Peguero struck out and when Stefen Romero lined another run-scoring single to right field, Wheeler was done, replaced by former Yankees left-hander Sean Henn.

Wheeler admitted the long wait in the top of the fifth affected him, and he also mentioned the temperature; at 67 degrees, it was far cooler than a Las Vegas home game.